Systems for producing nanopowders are known which ionize an inert gas to create a plasma in a reactor vessel that vaporizes a precursor material. A quench and/or reaction gas is injected into the vaporized precursor material to produce nanopowders having a desired composition. U.S. Pat. No. 5,514,349 discloses a transferred arc system similar to a tungsten inert gas (TIG) welder, wherein a single rod of metal precursor material acts as an anode, and is vaporized by feeding the anode past a nonconsummable tungsten cathode to expose the anode to a discharge arc. A gas in then injected into the vaporized material to quench and form the nanopowder. The transferred arc system is designed to avoid erosion of the tungsten electrode.
U.S. Pat. No. 6,472,632 discloses another method to produce nanopowders which uses a prior art axial electrothermal gun, as illustrated in FIG. 1. The axial electrothermal gun or axial gun 10 is shown with a breech electrode 11, an annular muzzle electrode 12, and a barrel 13 having a hollow bore 14. The breech electrode 11 fits into and pneumatically seals one end of the hollow bore 14. The muzzle electrode 12 is attached to and substantially axially aligned with the barrel 13. The breech electrode 11 further is connected by way of a conducting wire 15 to the negative terminal of a high-power, pulsed discharge power supply 16, the positive terminal of which is connected by way of a conducting wire 17 to the muzzle electrode 12. Unlike the transferred arc system, the polarity of the electrodes in the axial gun 10 is not important, and the device can be operated with the polarity reversed. This axial gun embodiment has been successful in producing moderate volumes of nanopowder in the 10–100 nanometer range.
In operation, the power supply 16 is energized to create an electric field between the breech electrode 11 and the muzzle electrode 12, and thereby discharge a high power pulsed arc 18 between the electrodes. The discharge of the pulsed arc 18 ablates the muzzle electrode 12, which is the primary source for plasma. More particularly, the material removal rate from the muzzle electrode 12 is a factor of 10–100 greater than the material removal rate from the breech electrode 11.
The pulsed arc discharge in the axial gun 10 occurs in an interval of time too short to allow the plasma to fully expand and equilibrate in the surrounding atmosphere (inertial confinement), and in an area physically confined by the walls of the bore 14 (physical confinement). The combined confinement creates a high temperature (˜50,000° K.), high density (˜1020/cm3) plasma which is not ordinarily obtainable in other plasma based systems such as those disclosed by U.S. Pat. Nos. 5,514,349; 5,874,684; and 5,851,507. The plasma acts to ablate the muzzle electrode 12 by means of the rapidly exiting high-temperature plasma. The plasma together with the ablated material exits the muzzle 12 under high pressure (˜15,000 PSI) and supersonic velocity. The ablated material thereafter may be quenched by and/or reacted with a surrounding gas such as one or more of air, oxygen, nitrogen, or helium to produce a cloud of nanopowder.
Referring to FIG. 2, a prior art transferred-arc-discharge process is illustrated in which a tungsten electrode 20 is shielded in a flow of pure inert gas 21 such as Argon, and is principally aligned with a rod 22 of precursor material. The inert gas shield protects the tungsten electrode 20 from erosion and oxidation. The inert gas ionizes to sustain the arc, but does not act to quench or react with material removed from rod 22.
Rod 22 is connected by way of a conducting wire 23 to the positive terminal of a DC power supply 24, the negative terminal of which is connected by way of a conducting wire 25 to the tungsten electrode 20. The tungsten electrode 20 is charged negatively with respect to the rod 22 to retard the absorption of heat and rate of erosion of the tungsten electrode. With these polarities, the material removal rate from the rod 22 is a factor of 100–1000 times greater than that of the tungsten electrode 20.
In operation, the DC power supply 24 is energized to effect a continuous DC low power arc discharge between the tungsten electrode 20 and the rod 22. The arc discharge erodes rather than ablates the rod 22. The material so produced is conveyed away from the vicinity of the arc discharge by the flow of the pure inert gas 21, and injected with a quench and/or reaction gas(es) 21, such as argon, helium and oxygen, to form the nanopowder.
From the above, it should be readily apparent that material removal in the operation of a transferred arc process, or an axial gun process is primarily from a single electrode.